Computer information processing and image data processing technologies have found their way into such fields as news printing and commercial printing, providing a method of drawing an image directly on a printing plate by means of a laser beam according to signals from the computer, without the necessity of photographic film. The printing plate known in the printing industry as PS plate, or pre-sensitized aluminum base plate, is widely used as a printing plate for the purpose.
In the new method, a drum-rotating system known as a facsimile apparatus has been used to draw an image on such a printing plate. Laser beams are applied in the process of image-drawing on the plate wound around the drum. To raise the drawing efficiency, a plurality of beams have come to be used rather than a single beam.
An apparatus such as that mentioned above for drawing an image on the printing plate as shown in FIG. 12 is known as multi-beam system. In FIG. 12, the reference numeral 2 indicates a horizontally positioned drum which is driven and rotated in the direction of an arrow a by a motor M. A printing plate 4 is wound on the outer circumferential surface of the drum 2. Since the printing plate 4 is wound in the form of a right circular cylinder with the starting sideline 6 and the end sideline 8 not shifting from each other, the top line 10 and the bottom line 12 are positioned in parallel to the direction of the drum axis.
Additionally, an optical unit 14 is provided with optical devices in which a laser beam emitting from a laser beam source 16, such as a semiconductor laser, passes through a multi-beam generating element 18 as grating and Wollaston prism to be divided into a multi-beam. This multi-beam then passes through a group of AOM's (acoustical-optical modulators) 20 switch-controlled according to image signals from a computer CM, enters an exposing means provided with a light source array 22 made up of optical fibers, and then is irradiated on the printing plate 4 through a lens 24.
This light source array 22 is one example of the exposing means to directly record an image on the printing plate. An image data is recorded on the printing plate 4 in the form of image band 26 with a band width B corresponding to the width of the multi-beam irradiation. A moving mechanism H is provided to move the optical unit 14 continuously in the direction of an arrow b at a uniform rate by a distance of the band width B per one rotation of the drum, that is, during the time in which drum 2 makes one revolution.
The prior art exposure apparatus has a serious shortcoming. As the drum 2 rotates at a constant velocity, the optical unit 14 or optical system including the exposing means moves continuously by a distance of the band width B for every revolution of the drum. That is, as shown in FIG. 13, the recorded image band 26 shifts by a distance equivalent to the band width B for every revolution of the drum and forms a spiral band around the printing plate 4. Unwound and opened, the printing plate 4 has a recorded image made up of a number of band strips, but the completed image frame is a parallelogram with the bottom line shifted from the top line by the band width B because the image band is recorded in the form of a spiral. The printing plate 4 with a recorded image frame 28 distorted with respect to the printing plate 4 is not suitable for printing.
If, for example, a 128-beam light source array 22 is adopted for a newspaper printing plate with a drum circumference of 46 inches and an image element density of 909 dots/inch, then the band width B, which corresponds to 128 image elements, is 128/909=0.14 inch or 3.6 mm. Because of the spiral recording, the top line 28c and the bottom line 28d of the recorded image 28 shift from each other by 3.6 mm, which shift is visually apparent. In addition, the angle .theta. of inclination, calculated from tan .theta.=0.14/46, is 0.17 degrees. That is, the starting sideline 28a and the end sideline 28b will have an inclination of 0.17 degrees, though they are parallel to each other.
As a way to avoid this distortion of the Image, an intermittent moving method as outlined in FIG. 14 was proposed. In this method, the optical unit 14 is left standing while the drum 2 makes a revolution for recording the image. Thereafter, the optical unit 14 is moved by a distance of the band width B by means of a moving mechanism H. The time for the movement has to be set to an integral multiple of the time needed for the drum to make a revolution. This intermittent moving system allows the image band 26 to form a right cylinder shape without spiralling. Thus the image 28 is recorded on the printing plate 2 in the form of a right quadrangle without distortion. The problem is, however, that intermittent moving lengthens the total exposure time significantly, lowering the efficiency of plate-making work. Furthermore, repeated moving and stopping of the heavy optical unit makes the apparatus complicated and expensive, and can cause vibration, leading to deterioration of image quality and to mechanical trouble.